Ahead of time.... you need to find out, when... the various summer venues/programs start advertising their enrollment for summer programs and the deadlines. Because, the programs fill up fast.
Research the summer programs in your city and see what is available and the time lines.
Not all summer programs, are the same length of time.
'Summer camps' are not ALL summer long, for 2+ months or more.
Not in my area.
And 'summer camps' are not all the same. Nor are the hours.
Some are 1 month. Some are a couple of weeks. Some may be for 2 months. But again, not all summer programs are the same.
And there are private organizations that have summer programs, and then there are "city" sponsored summer programs. And then there are places like the YMCA that have various programs too, and per age etc.
So you need to research it. Call the places. And see. Get information.
Or, you hire a Nanny, to watch your 3 kids. And do things w/them and have... a routine with them.
Typically, per the families I know, this is what they do:
depending on their budget and how many kids they have, they either enroll their kids in various summer programs run by private schools or organizations. Or send their kids to their city and county "summer" fun type programs. Because, at least in our city, the city and county summer programs, also may have, After Care... for kids who need to stay longer than the end-time of the summer program. Here, for example, the city and county summer program, which is daily... starts at 8:30 and ends at 2:00pm. It is not all day. And it is NOT ALL summer long until the kids go back to school.
And, no matter who you hire, to watch your kids... you need to make sure they are background checked, have a valid drivers license since they will probably be expected to transport... your children or drive them places, have CPR and/or First Aide training, be clear of T.B. etc., and are licensed. It depends on how, professional or not, you want your "babysitter" to be and their experience and background. And if you expect them to keep house and do house cleaning plus, watch your kids, then that is something else.
What are your work hours?
Thus, this will determine how many hours you need a babysitter for, in your home. Or if you need to take them, to the babysitter.
And, what do you expect the babysitter/Nanny to do with them to "keep them busy?"
You have to have a 'contract' with them.... and determine what you will provide or not, to the Nanny/babysitter. Gas money, food, incidentals etc. for your kids. And do you expect the Nanny to also "tutor" your older kids to keep them abreast with their school skills or not?
Or just play with them and be a companion?
And per naps, and feedings, how will it be for your children?